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POHow to synchronize handling of filesystemwatcher events?
 

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  1. POHow to synchronize handling of filesystemwatcher events?
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    <p>I have the following synchronization problem.</p> <p>A thread runs in a loop and if it detects a folder in filesystem which was not processed yet (it may have been added while the app was not running), the folder is processed as a new folder by <code>Process</code> method: </p> <pre><code>void Loop() { while (run) { lock(renameLock) { var newFolders = EnumerateNewFolders().ExceptThoseMarkedAsRenamed(); Process(newFolders); MarkAsProcessed(newFolders); } Sleep(); } } </code></pre> <p>Renamed folders should not be processed as new but rather processed as renamed and marked as renamed so that they are not processed as new later.</p> <p>To take care of this, I have a <code>FileSystemWatcher</code> and a handler for the <code>Renamed</code> event:</p> <pre><code>private void fsw_Renamed(object sender, RenamedEventArgs e) { lock(renameLock) { ProcessRename(e); //mark folder as renamed so that ExceptThoseMarkedAsRenamed filters it out MarkAsRenamed(e); } } </code></pre> <p>Usually this works and when a rename is performed, the folder is first marked as renamed in <code>fsw_Renamed</code> and then filtered out in the loop by <code>ExceptThoseMarkedAsRenamed</code> and not processed as new.</p> <p>But sometimes the following order occurs:</p> <ol> <li>Folder is renamed in Windows Explorer and in the filesystem (while the Loop is holding lock)</li> <li>fsw_Renamed occurs, but cannot obtain the lock</li> <li>The looping thread is still holding lock and picks up the folder as new (it was not marked as renamed yet)</li> <li>The looping thread releases the lock</li> <li>Renamed event occurs, gets the lock, and folder is marked as renamed. But it is already too late since it was already processed as new.</li> </ol> <p>I am having trouble figuring out how to ensure that a renamed folder is never processed as new. </p> <p>A renamed folder is such folder for which the <code>Renamed</code> event <strong>occurred or will occur in near future</strong>, because there is an inherent delay between filesystem change and the <code>FileSystemWatcher.Renamed</code> event (and the actual change in filesystem and the event raised are not atomic).</p> <p><strong>How to ensure that the change done by fsw_Renamed is always taken into account by the Loop, even if the event occurred while Loop was holding the lock?</strong></p>
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    1. COI can't see all your other code but on the outset, this doesn't look like a very efficient way to deal with the scenario. You look like you'll have HUGE lock contention. Are you able to 'signal' your first thread to deal with the changes (instead of sleep())? I might be missing why you're looping at all.
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      1. POHow to synchronize handling of filesystemwatcher events?
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    2. COthe code I have posted is a heavily simplified sample and I am hot having any problems with lock contention. There is no real sleep in the Loop. You are also not answering my question, only commenting on an unrelated issue. I am looping because I need to detect new folders and I need to process also folders which were not handled by FSW (e.g. while the app was not running or it was in suspended state).
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